The present invention relates generally to the art of cleaning workpieces such as semiconductor wafers during various stages in the manufacturing process of integrated circuits, and more particularly, relates to an improved method and apparatus whereby a freely rotating wafer is cleaned between two adjacent motor driven brushes wherein there is a substantially uniform relative velocity distribution between the top brush and the wafer""s surface.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder.
Several of the processes used to manufacture semiconductor wafers introduce particles or contaminates to the surfaces of the wafer. For example, chemical-mechanical polishing (CMP) involves placing the wafer on a polishing pad in the presence of slurry. Slurry typically contains chemicals that etch away material from the wafer""s surface and abrasive particles that assist in mechanically removing material from the wafer""s surface. Slurry may contain, for example, KOH and colloidal or fumed silica abrasive particles. The wafer is then pressed against the polishing pad and relative motion is created between the wafer""s surface and the polishing pad to remove material from the wafer""s surface. The wafer""s surface is thereby exposed not only to the chemicals and particles contained in the slurry, but also to material removed from the wafer""s surface. The process of pressing and causing relative motion between the wafer""s surface and these contaminates undesirably adheres the contaminates to the wafer""s surface.
The wafer""s surface on which integrated circuitry is to be constructed must be extremely clean in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. The material layers (deposited thin film layers, usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made extremely clean to avoid contamination of the circuitry.
Conventional post-CMP wafer cleaning commonly uses a combination of buffing, double-sided brush scrubbing, megasonic cleaning and spin-rinse drying to remove contaminates from the wafer""s surface. The present invention relates to double-sided brush scrubbing so buffing, megasonic cleaning and spin-rinse drying, all presently known cleaning techniques, will not be explained in detail so as to not obscure the present invention.
One conventional style of double-sided brush scrubbing is accomplished by placing a freely rotating wafer between the working surfaces of a top and a bottom brush. In conventional cleaning, the top and bottom brushes are rotated by a motor in the same direction and at the same speeds and have the same pattern of raised features on their working surfaces. The motorized rotating top and bottom brushes grip the edge of the freely rotating wafer and cause the wafer to also rotate in the same direction as the brushes.
Applicant has discovered that in conventional cleaners the top brush commonly rotates at about twice the speed of the wafer resulting in a nonuniform relative velocity between the top brush and the wafer across the wafer""s surface. The consequences of applicant""s discovery are graphically illustrated in FIGS. 2a and 2b. The velocities at various points across the rotating wafer 100 and rotating top brush 101 are represented by the arrows between lines C2 and C3 and lines C1 and C3 respectively. The line C1 representing the speed of top brush 101 is steeper than the line C2 representing the speed of the wafer 100 since, as previously mentioned, the top brush 101 commonly rotates at about twice the speed of the wafer 100. The relative velocity between the top brush 101 and the wafer 100 may be found by subtracting their individual velocities at various points. The relative velocity between the top brush 101 and the wafer 100 is illustrated in FIG. 2b. Specifically, applicant has discovered that as the wafer 100 rotates in a conventional dual brush cleaning system, the relative velocity between the top brush 101 and the wafer 100 is higher near the center and lower near the edge of the wafer 100.
Applicant has further discovered that conventional dual brush cleaning systems typically remove particles satisfactorily from the center of the wafer 100, but often leave a band of contaminates near the edge of the wafer 100. Applicant has thus discovered a need for a dual brush cleaning system with a high uniform (across the entire width of the wafer 100) relative velocity between the top brush 101 and the wafer 100.
What is therefore needed is an apparatus and method of cleaning wafers that produce a high uniform relative velocity between the top brush and the wafer""s surface.
Therefore it is an object of the present invention to provide an apparatus and method for cleaning workpieces that addresses and resolves the shortcomings of the prior art described above. Another object of the present invention is to provide a dual brush cleaning system where all points on the workpiece""s surface experience substantially similar top brush velocities during the cleaning process as the workpiece rotates between the brushes.
The apparatus portion of the present invention relates to a dual brush cleaning system for cleaning a workpiece""s surface. The cleaning system has two brushes, a top and a bottom, each with a working surface on one end and is advantageously connected to a shaft on the other end. The working surfaces are positioned opposite of each other and spaced apart enough for a workpiece to be placed, gripped and cleaned between them. The shafts for both brushes may be connected to means, such as one or more motors, for rotating the brushes at separate speeds.
The apparatus is preferably able to create the condition of providing a uniform relative velocity between the top brush and the workpiece across the width of the workpiece as the workpiece rotates between the brushes. Sensors may be inserted to monitor in real-time the rotational speeds of the top brush and the workpiece and to then adjust the speed of the bottom brush as needed. However, the cleaning system may be simplified by determining through empirical means the bottom brush speed necessary to obtain a uniform relative velocity between the top brush and the workpiece. Once the desired bottom brush speed has been determined that produces the desired results for a particular application, the bottom brush speed may simply be set at this level.
While the top brush is used primarily to clean the workpiece""s top surface, the bottom brush is used primarily (although it also cleans the workpiece""s bottom surface) to rotate the workpiece. It is thus important for the working surface of the bottom brush to have an efficient grip on the workpiece to more efficiently rotate the workpiece. Applicant has discovered that a bottom brush with a plurality of nubbs (raised areas) on its working surface may be used to efficiently grip the workpiece and provide the necessary rotational motion to the workpiece.
The method portion of the present invention may be practiced in a dual brush cleaning system by inserting a workpiece, preferably a semiconductor wafer, between the working surfaces of a top and bottom brush. The brushes may be rotated after the workpiece has been inserted, but are preferably already rotating at the speeds necessary to produce a substantially uniform relative velocity between the top brush""s working surface and the workpiece""s surface.
These and other aspects of the present invention are described in full detail in the following description, claims and appended drawings.